It is well known that oil and gas deposits are contained in subterranean earth formations and that boreholes are drilled into these formations for the purpose of recovering these petroleum deposits. During the drilling operations, it is common to pump a drilling fluid, or drilling mud, into the borehole through the drill string to lubricate and cool the bit, to maintain hydrostatic pressure head in the borehole to overbalance the subterranean formation pressures, and to carry the drill cuttings from the bit to the surface of the earth.
It is also well known that subterranean formation pressures generally increase with depth. Low permeability formations, such as shales, exhibit a pressure that is a measure of the pressure exerted by fluid trapped within non-interconnected interstices or pores of the formation. The measure of this pressure is commonly called "formation pore pressure." In permeable formations the exhibited pressure is a measure of the fluid trapped within the interconnected interstices or pores of the formation, and is generally referred to as "formation pressure." Further, it is generally known that low-permeability formations, such as shales, commonly overlie abnormally high-pressured fluid within the porous formation.
A problem in all oil and gas well drilling operations is the maintenance of sufficient hydrostatic pressure head of drilling mud to overbalance the subterranean formatin pressure at the bottom of the borehole. A pressure overbalance or "bottomhole pressure differential" must be maintained in order to prevent high-pressured fluids within porous formations from being released through the borehole to the surface. An uncontrolled release of high pressured fluid from within the formation through the borehole is commonly referred to as a "blowout". A blowout can cause irreparable damage to the borehole and surface equipment and death and injury to drilling personnel located near the surface drilling equipment.
Excessive hydrostatic pressure head, together with additional pressure due to friction while circulating the drilling mud or while lowering the drill string into the borehole, can cause the formation to be fractured with possible resultant loss of mud to the surrounding formation. Thus, maintenance of a proper bottomhole pressure differential, i.e., overbalance, is important to well safety. However, this is difficult since the pressure varies with the drilling mud being used and the formation being encountered. Exact knowledge of formation pressure is necessary but is not easily obtained. Generally accepted practice requires the removal of the drill string and the running of a wireline log to determine the bottomhole pressure differential with the resultant loss of time and expenditure of money.
A general object of this invention is to provide an improved system that may be used in connection with downhole testing during drillling operations, wherein it is possible to measure formation pore pressure without removing the drill string from the hole.
Still another object is to provide an improved system for measuring formation pressures with accuracy.
Yet another object is to provide apparatus for obtaining the pressure measurements of subsurface earth formations in connection with surface drilling operations wherein a minimum amount of rig time is lost.
Other objects and features of the invention will become apparent upon consideration of the following description thereof when taken in connection with the accompanying drawings.